rcu: Avoid running boost kthreads on isolated CPUs

When the boost kthreads are created on systems with nohz_full CPUs,
the cpus_allowed_ptr is set to housekeeping_cpumask(HK_FLAG_KTHREAD).
However, when the rcu_boost_kthread_setaffinity() is called, the original
affinity will be changed and these kthreads can subsequently run on
nohz_full CPUs.  This commit makes rcu_boost_kthread_setaffinity()
restrict these boost kthreads to housekeeping CPUs.

Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>